In communication networks that use orthogonal frequency division multiplexing (OFDM), the bandwidth is partitioned into multiple subcarriers. Similarly, multiple transceivers can share the same transmission bandwidth using networks with orthogonal frequency-division multiple-access (OFDMA). OFDMA, also partitions the bandwidth into subcarriers, and transceive data concurrently using non-overlapping subsets of the subcarriers.
OFDM and OFDMA networks offer high spectrum efficiency and low complexity implementation and are used in networks designed according to numerous wireless standards, such as IEEE 802.11, IEEE 802.16 and 3GPP-LTE. In OFDM and OFDMA networks, the bandwidth available for communications is a set of N parallel channels or subcarriers. As defined herein, a carrier or a subcarrier is a frequency which carries analog or digital signal. A single OFDM symbol includes multiple subcarriers and each subcarrier carries data, which is represented as modulation alphabet based on a digital modulation scheme, such as quadrature amplitude modulation or phase-shift keying.
In ideal conditions, the subcarriers remain orthogonal to each other in the frequency domain within the received signal, i.e., the energy of one subcarrier is not present in the received signal for another subcarrier.
However, frequency orthogonality is often destroyed by Doppler frequency shift and carrier frequency offset (CFO) between of the transmitter and the receiver. In such cases, the signal from one subcarrier interferes with the signal of other subcarriers as inter-carrier interference (ICI).
FIG. 1A shows an energy distribution of subcarriers, i.e., SC−1 101, SC0 102 and SC+1 103 as a function of frequency, normalized to the subcarrier spacing when CFO is absent. The energy of the subcarrier SC0 reaches a maximum 104 at frequency f0. The energy 105 of the signals of subcarriers SC−1 and SC+1 at the frequency f0 is zero because the frequencies are orthogonal.
FIG. 1B shows the energy distribution of subcarriers subject to the CFO 120. The energy of subcarrier SC−1 and SC+1 at frequency f0 are non-zero. That is, the energy from the subcarriers SC−1 and SC+1 becomes interference to subcarrier SC0.
The ICI can be quantified by the ratio of the received energy from the subcarrier to the total energy from all other subcarriers, i.e., the carrier-to-interference ration (CIR). The CIR decreases as the CFO increases. ICI can increase the bit-error-rate (BER), and in severe cases make the network inoperable.
ICI Reduction
The effect of ICI can be reduced using a number of conventional methods, such as frequency domain equalization (FDE), transmit signal windowing, and self-canceling ICI.
The FDE-based method estimates the CFO. Then, a matrix is constructed using the estimated CFO. Applying the matrix to the received signal in the time domain reduces the ICI. However, the CFO compensation requires an accurate estimate of the CFO, which increases noise and complexity of the receiver. The computation complexity of the matrix is high.
The transmit signal windowing performs oversampling of the received signal in the frequency domain. The oversampled signal is filtered in the time domain by a low-pass filter, such as Hanning window.
The self-cancelling scheme maps each transmit data symbol to an adjacent pair of subcarriers with opposite polarity, such that α2k=−α2k+1 where α2k is the data on the 2kth subcarrier. The interference to the mth subcarrier, (m≠2k or 2k+1), from 2kth and (2k+1)st subcarrier partially cancels each other.
OFDM/OFDMA Transmitter and Receiver
FIG. 2 shows a conventional OFDM transmitter 200 and OFDM receiver 250 with built-in frequency diversity communicating a signal 230 via a wireless channel h 299. The received signal 251 includes noise n(t) 298.
In the transmitter 200, input data (symbols) 205 are partitioned into blocks A 201 by a serial to parallel (S/P) module 225. Each block A is a vector of length M and each element am of the vector is a complex number having values determined by the input data and a modulation format.
An interleaved diversity module 221 adds frequency diversity by generating a interleaved diversified block B 202 including elements bm,k based on the block A 201. For example, the diversity module repeats the block A K times to produce the interleaved diversified block B. A mapping module 222 performs tone mapping, pilot and guard band null tone insertion to produce N-point frequency domain symbols S 203. The symbols S are converted to time domain OFDM symbols s 204 using an N-point inverse discrete Fourier transformation (IDFT) 223. The OFDM symbols s are ordered serially 224 by a parallel-to-serial module. A cyclic prefix (CP) is appended to produce a baseband signal s′(t) 206. The baseband signal is up-converted 207 to the radio frequency (RF) signal sRF(t) 230 and transmitted over the channel 299.
The received signal rRF(t) 251 is down-converted and discretized to a discrete baseband signal rBB(t) 256. A serial to parallel module 271 removes the cyclic prefix from the signal rBB(t) producing a signal r(t) 252, which in turn is converted by a DFT block 272 to frequency-domain symbols R 253. A demapping module 273 removes non-data tones and passes data symbols Y 254 to a subcarrier combining module 274. The module 274 outputs data Z 255, where each element in Z, Zm, is produced based on the combined signal from multiple subcarriers. The output Z 255 is assembled into a continuous output stream 256.
The transmitter and receiver provide improved performance due to the frequency diversity. However, the transmitter and receiver do not improve the performance with respect to carrier frequency offset (CFO).
Accordingly, it is desired to improve the performance of OFDM/OFDMA networks performance under CFO.